The present invention concerns a control device for a pressure medium servomotor of the axial piston type, and, more particularly, to a control device which positions an axial piston of a servomotor in a central stroke position both simply and reliably.
In a known control device for a servomotor driven by compressed air as shown in FR-PS 13 97 486, two central position control connections are located at a greater distance from one another than the control edges. Thus, in the central stroke position of the working piston, the control connections are each connected openly to an associated working pressure chamber. The two central position control connections are connected via a selector shut-off valve to an intrinsically closed expansion vessel such that, in the central stroke position of the working piston, the working pressure chamber connected to the pressure source is connected openly to the expansion vessel, whereas the other working pressure chamber is shut off relative to the selector shut-off valve. A catch must be used in order to be able to position the working piston in the central stroke position. This arrangement also has to fulfill a braking function for the working piston, in addition to the retention function in the unpressurized condition, because braking cannot be achieved by the working pressure alone. In this known device, it is also necessary to switch off the working pressure in good time on reaching the central stroke position before the expansion vessel is filled because otherwise the working piston starts moving again and leaves the central stroke position. An expansion vessel with an infinitely large volume or the relief of the associated pressure conduit to atmosphere would have the result that the working piston could no longer leave the central stroke position.
In another known control device for controlling a pressure medium servomotor of the axial piston type, as shown in DE-PS 712 340, two axially adjacent annular control chambers concentric with the piston axis are formed at the periphery of the working piston to position it in two different central stroke positions. Each control chamber is connected via a control hole in the working piston to the respective working pressure chamber adjacent to the other control chamber. Consequently, the configuration of the working piston and the pressure supply to the working cylinder are complicated, as is the configuration of the valve controlling the pressure supply.
Yet another type of control device for controlling the stroke position of a pressure medium servomotor of the axial piston type is shown in U.S. Pat. No. 3,312,146. The working piston is additionally fixed in its stroke central position by two auxiliary pistons which are respectively located in one of the two working pressure chambers through which the through piston rod of the working piston passes. The auxiliary pistons are located so as to be displaceable relative to both the working cylinder and relative to the working piston so that the working piston is supported on the working cylinder in its end-of-stroke positions by the respectively associated auxiliary piston (pressure-relieved by way of a 5/3-way control valve), whereas it is supported in its stroke central position by the auxiliary piston then subjected to pressure via the control valve. The other auxiliary piston located in the end-of-stroke positions in the respective working pressure chamber subjected to working pressure is in contact, decoupled from the working piston, with a housing stop fixing the stroke central position. A control edge on the auxiliary piston shuts off relative to its working pressure chamber the associated central position control connection of the working cylinder. This connection is continually connected to the pressure-relieved return conduit and bypasses the control valve. The auxiliary pistons increase the design complexity and reduce the useful stroke of the working piston and the useful cross-section of the working cylinder.
An object on which the present invention is to configure a device for controlling the hub position of a pressure medium servomotor such that a servomotor of the axial piston type driven by hydraulic working medium can be positioned in a central stroke position by simple but functionally reliable means.
This object has been achieved in an advantageous manner by an arrangement of the control edges of the working piston relative to the central position control connections such that, in the stroke central position of the working piston, both central position control connections are shut off or throttled relative to the working pressure chambers. The pressure conduit used as the return to the pressure-relieved reservoir is connected to the shut-off valve as the common pressure conduit leading to a pressure-medium vessel. In the first valve position of the shut-off valve, both central position control connections are connected to the pressure conduit used as the return. In the second valve position of the shut-off valve, both central position control connections are shut off relative to the pressure conduit used as the return. Selector valve apparatus are provided for the stroke central position, and these apparatus make it possible to connect both end position control connections synchronously to the pressure source.
The device according to the present invention is particularly characterized by a simple and functionally reliable control with respect to the approach to and retention of the stroke central position by the working piston. In the device according to the present invention, the two end position control connections are synchronously subjected to pressure while both central position control connections are opened to atmospheric pressure. If the working piston is located in one of the end-of-stroke positions, the pressurized oil supplied, via the end position control connection associated with the other end-of-stroke position, is led off via one or both central position control connections to the pressure-relieved return. Because of the resulting pressure difference in the two working pressure chambers acting on the working piston, the latter is displaced until it closes the two central position control connections by way of its control edges, and pressure equilibrium occurs at the working piston. The pressure forces of the two working pressure chambers hold the working piston in its stroke central position.
The throttle resistances located in the region of the control connections in the pressure conduits are matched for the stroke motions of the working piston between the three positions at which it can be located.
The pressure at the end position control connections can be switched off after an arbitrary period because of the catch arrangement on the working piston. This has the advantage that the complete gear-change control system is unpressurized and therefore has no leaks.
In the device according to the present invention, only three pressure conduits between the working cylinder and the selector valve have to be controlled because the two central position control connections can be connected by a common pressure conduit which leads to the selector valve. No special control chambers are thus necessary on the working piston to locate the stroke central position.